The present invention generally relates to image forming apparatuses and more particularly, to a multi-color image forming apparatus such as a multi-color copying apparatus, a multi-color printer or the like.
Recently, as one type of a multi-color image forming apparatus, there has been provided a multi-color copying apparatus in which a plurality of developing devices containing developers of different colors, respectively are disposed around a photosensitive drum and are driven sequentially such that an image of a desired color is obtained.
However, this known multi-color copying apparatus has the following drawback due to the fact that an electrostatic latent image formed on a surface of the photosensitive drum is passed by fronts of all the developing devices. Namely, when the electrostatic latent image is developed by using a downstream one of the developing devices, the electrostatic latent image is initially brought into contact with the developers of upstream ones of the developing devices if the developers of the respective developing devices are held in contact with the surface of the photosensitive drum, thereby resulting in mixing of the colors.
Conventionally, in order to prevent such a phenomenon, there have been proposed methods that a mechanism for retracting an unused one of the developing devices is provided and that drive of the unused one of the developing devices is stopped. However, in the case where the mechanism for retracting the unused developing device is provided, such problems arise that the unused developing device cannot be positioned accurately, that the developer leaks out of the unused developing device due to an impact force exerted at the time of displacement of the unused developing device and that a construction of the copying apparatus becomes complicated and thus, the assembly steps and the number of the components are increased, thereby resulting in poor production efficiency and high production cost of the copying apparatus. Meanwhile, in the case where drive of the unused developing device is stopped, such an inconvenience is incurred that when drive of the unused developing device is merely stopped, the developer remaining in a portion of the unused developing device, which portion confronts the photosensitive drum, adheres to the electrostatic latent image, thereby resulting in mixing of colors.